1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation. In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing or increasing a hydraulic braking pressure supplied to a wheel brake cylinder. In view of the fact that when the hydraulic braking pressure supplied to the wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before a coefficient of friction of a road surface relative to the road wheel reaches a maximum, the anti-skid control system controls the hydraulic braking pressure in response to the variation of wheel speed and that of wheel acceleration in order that a slip rate of the road wheel results in around 10% to 20%, that is, the maximum coefficient of friction is obtained.
In connection with the anti-skid control system as described above, Japanese Patent Laid-open Publication No. 49-56083 discloses an anti-skid control system provided with a pair of inlet valves disposed between a pair of wheel brake cylinders mounted on either sides of a vehicle and a master cylinder for applying a hydraulic braking pressure to the wheel brake cylinders. This anti-skid control system is arranged to increase gradually the hydraulic braking pressure in the wheel brake cylinders by energizing or de-energizing the inlet valves simultaneously.
In the prior system which simultaneously increases the hydraulic braking pressure supplied to a pair of wheel brake cylinders with a pair of inlet valves energized or de-energized simultaneously, there is no lag between their pressure increasing timings, so that the characteristics such as the increasing speed of the hydraulic pressure and etc. are stable. However, the hydraulic pressure in the master cylinder is decreased largely when the hydraulic pressure in the wheel brake cylinders is increased, so that the depressed amount of the brake pedal comes to be large to thereby injure a pedal feeling, i.e., the feeling in braking operation.
Also, there is known an anti-skid control system which increases the hydraulic braking pressure in a wheel brake cylinder mounted on each of the road wheels without controlling a timing for increasing the hydraulic braking pressure. In this system, therefore, the timing for increasing the pressure in one of the wheel brake cylinders may coincide with the timing for increasing the hydraulic braking pressure in another of the wheel brake cylinders, or may not coincide therewith. In the case where the coincidence occurs, the hydraulic braking pressure in the master cylinder is decreased largely to thereby enlarge the depressed amount of the brake pedal comparing with the case where it does not. In the case where the coincidence does not occur, the hydraulic braking pressure in the master cylinder is decreased small, so that the depressed amount of the brake pedal is small. Consequently, a bad pedal feeling is caused, which is not appropriate for a feeling in braking operation. Further, the increasing speed of the hydraulic pressure will vary between the cases where the timing for increasing the hydraulic braking pressure in one of the wheel brake cylinders coincide with that in the other thereof, or where the former does not coincide with the latter, so that it is difficult to effectuate a desired hydraulic braking pressure control.